[administrativa] no more anonymous bug reports

I'm sorry, but I feel that in order to keep a high quality in the bug tracker,
I've had to disable the ability to submit info without being logged in to
sourceforge. It has become a little too common that people pop up a very old
bug report and fill in spam as comments to them.

Fortunately, sourceforge is a huge site and very many people already have
accounts there - and getting a new one is easy, swift and could be used for
all sorts of other services as well. Also, getting logged in users to submit
reports will hopefully also make it easier to get feedback and follow-up
comments to bug reports, which tend to be very hard to get from the drive-by
anonymous posters who never return to check whatever happened to their bug
report...